1. Field of the Invention
The present invention relates to a method of making a leadframe used for fabricating a semiconductor device. It also relates to a method of making a semiconductor device by using such a leadframe.
2. Description of the Related Art
FIG. 19A of the accompanying drawings shows a conventional surface-mounting semiconductor device. As illustrated, the device, generally indicated by reference numeral 9, includes a first conductor 90, a second conductor 91 and a semiconductor chip 92. The first conductor 90 has a lower surface (terminal surface) 90a and an upper surface (bonding surface) 90b. Likewise, the second conductor 91 has a lower surface (terminal surface) 91a and an upper surface (bonding surface) 91b. The first conductor 90 generally consists of two portions, i.e., a thinner portion 90c and the remaining thicker portion, which corresponds in position to the terminal surface 90a. 
The semiconductor chip 92 is formed with a lower and an upper electrodes (not shown) on its bottom and head surfaces, respectively. The chip 92 is mounted on the first conductor 90, with its lower electrode electrically connected to the bonding surface 90b. The upper electrode of the chip 92 is connected to the bonding surface 91b of the second conductor 91 via a connection wire 93. The chip 92 and the wire 93 are entirely covered by a resin package 94. On the other hand, the terminal surfaces 90a, 91a of the first and the second conductor 90, 91 are exposed in the bottom surface 94a of the package 94.
The semiconductor device 9 is fabricated in the following manner. First, a suitable electroconductive frame (leadframe) is prepared. This frame includes portions to be used as the first and the second conductors 90, 91. The semiconductor chip 92 is mounted on the frame, and then the bonding of the wire 93 is performed. The mounted chip 92 and the wire 93 are enclosed by a resin material. Finally, the thus obtained intermediate product is diced into smaller pieces, one of which provides the semiconductor device 9 shown in FIG. 19A.
As noted above, the first conductor 90 of the device 9 is formed with a thinner portion 90c. This nonuniform thickness results from the corresponding configuration of the leadframe used for making the device 9. Conventionally, such a leadframe is produced from an electroconductive strip by etching. Specifically, referring to FIG. 20A, a mask 96 is formed by e.g. photolithography on the upper and lower surfaces of an electroconductive plate 95 of an uniform thickness. The mask 96 is formed with openings 97A and 97B corresponding in position to the prescribed regions to be etched. Upon application of an etchant, as shown in FIG. 20B, the prescribed portions of the plate 95 are etched away from above and/or below by half the thickness of the plate 95. As a result, some through-holes 99 are formed in the plate 95 at the places where the upper and the lower openings 97A, 97B overlap, whereas some xe2x80x9chalf-depth dentsxe2x80x9d are formed at the places where only the lower openings 97B are provided. Due to the provision of the half-depth dents, the plate 95 is formed with thin-walled portions 98 whose thickness is generally half the original thickness of the plate 95.
The conventional leadframe fabrication method has the following drawbacks.
The first problem is that the above etching process is rather difficult to perform on a hoop material. Thus, before a mask-forming process and etching process are carried out, the hoop needs to be cut into shorter pieces so that proper etching result can be obtained. Apparently, this additional cutting procedure diminishes the production efficiency.
The second problem is that the mask-forming process is time-consuming. Also, after the etching process is over, the mask 96 formed on the plate 95 needs to be removed. This results in a decrease in efficiency and an increase in cost.
The third problem is that the required configuration of the parts of the leadframe often fails to be obtained by etching. Referring to FIG. 19B, which is a plan view showing the first conductor 90 and the semiconductor chip 92 mounted on the conductor 90, the first conductor 90 is provided with round corners resulting from inevitable spreading of the etchant, even when use is made of an etching mask formed with a completely rectangular opening in it. Unfavorably, the round-cornered first conductor 90 has a smaller effective bonding area for the chip 90 than otherwise. This means that the marginal portion 90d of the conductor 90 is not used for any productive purposes but merely cause an increase in overall size of the semiconductor device 9.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a fabrication technique, whereby a compact semiconductor device can be produced efficiently and at low cost.
According to a first aspect of the present invention, there is provided a method of making a semiconductor device. The method comprises the steps of: mounting a semiconductor chip on a leadframe; producing an intermediate product by forming a packaging layer to enclose the chip, wherein the intermediate product includes the leadframe, the chip and the packaging layer; and cutting the intermediate product. The cutting step is performed by using a first cutter of a first thickness and a second cutter of a second thickness greater than the first thickness. The first cutter is used for making a xe2x80x9cfull cutxe2x80x9d in the leadframe, while the second cutter is used for making a xe2x80x9cpartial cutxe2x80x9d in the leadframe. In this specification, the xe2x80x9cfull cutxe2x80x9d implies that the leadframe is cut through in its thickness direction. The xe2x80x9cpartial cutxe2x80x9d, on the other hand, implies that the leadframe is not cut through in its thickness direction (nor in any other directions) and the depth of the cut is smaller than the thickness of the leadframe. The determination of where the full cut and the partial cut are to be made is based on a common cut line, so that the full cut and the partial cut coact to prevent the formation of burrs at the cut site.
The partial cut may be made after or before the full cut is made.
Preferably, the method of the present invention may further comprise the step of preparing the leadframe by processing an electroconductive material of a uniform thickness. In this case, the leadframe-preparing step may include sub-steps such as first punching, stamping and second punching. The first punching may be performed for forming a patterned region in the electroconductive material, the stamping may be performed for compressing the patterned region to provide a stamped portion of a thickness smaller than the uniform thickness of the electroconductive material, and the second punching may be performed for removing an unnecessary part from the stamped portion.
Preferably, the first punching may produce a xe2x80x9clandxe2x80x9d and an opening adjacent to the land. In this arrangement, when the subsequent stamping compresses the land, this land is allowed to spread out into the opening. This is advantageous to preventing unfavorable stress due to the stamping from remaining in the leadframe.
Preferably, the packaging layer may be formed in a manner such that it covers the stamped portion entirely but allows part of the leadframe to be exposed.
According to a second aspect of the present invention, there is provided a method of making a leadframe for fabricating a semiconductor device. The method may comprise the steps of: performing first punching on an electroconductive material of a uniform thickness; stamping the electoroconductive material to form a stamped portion of a thickness smaller than the uniform thickness of the electoroconductive material; and performing second punching on the electroconductive material to remove an unnecessary part from the stamped portion.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.